Turns out I didn't install the wire tap properly, thus current couldn't flow between the green and red/black wire. Once I fixed the wire tap, it worked as expected.

I also figured out the answer to several of my other questions,

What does the green wire do?

Holds a 3V signal when the stereo is on. When the green wire is grounded, some stereo functionality will be opened up. You can test this by touching the green wire to the chassis and watch some stereo buttons fade in and out.

What does the red/black wire do?

It carries a 12V signal when the car's key position is at on not when it's at acc.

How does the parking brake switch work?

First, notice that there is only one wire coming out of the switch. Where's the ground wire? Well, the switch is grounded directly to the chassis of the car so there is no need for a ground wire.

When the hand brake is down, it pushes on a little black pin that completes the circuit, grounding the red/black wire.

In other words, when the hand brake is down, the red/black wire is at 0V, when it's up, the red/black wire is at 12V.

What is the high-low-high signal required by the Pioneer?

Note: It's my understanding that only newer Pioneer models require the high-low-high signal, so if you have an old model maybe this doesn't apply to you

It's been suggested that simply grounding the green wire is all you need to do to get the stereo to function properly. Although this will work in some cases (in the limited testing I did), it will fail to provide the high-low-high signal when needed, thus disabling some stereo functionality .

In some cases when starting up the stereo, the Pioneer will give a prompt saying that you must pull the hand brake up, down, then up again, in order to use a particular feature. This is the high-low-high signal the Pioneer needs.

• Hand brake up -> circuit open -> green wire not grounded -> high signal
• Hand brake down -> circuit closed -> green wire grounded -> low signal
• Hand brake up -> circuit open -> green wire not grounded -> high signal